Implantable cardioverter defibrillators (ICDs) have traditionally been used in patients who survived, or have a high risk of experiencing, a sudden cardiac death event. More recently, indications have been expanded to include patients who have had asymptomatic nonsustained ventricular tachycardia, for example, with decreased ventricular function. Often, an ICD lead is placed in a patient's right ventricle where it provides for sensing of ventricular rate and/or other information for detection of sustained ventricular tachyarrhythmias. Some systems also include an atrial lead, which can, for example, sense atrial activity. Knowledge of both atrial and ventricular information can allow for a comparison of atrial beats in relation to ventricular beats, which, in turn, can help distinguish ventricular tachycardia rhythms from supraventricular tachycardia rhythms. For example, with ventricular tachycardia and complete atrioventricular disassociation, the atrial rate would generally be much lower than the ventricular rate.
Once an ICD device detects a ventricular tachycardia or a ventricular fibrillation, a delay typically occurs wherein the device charges a shock capacitor. The device then discharges the capacitor to deliver a shock, typically of approximately 25 J. An ICD device may also repeat the charge and discharge cycle (e.g., for approximately 5 cycles). Other ICD devices may provide programmable low-energy cardioversion in addition to high-energy shocks. Yet other ICD devices provide a feature that is commonly referred to as “tiered therapy”, which typically includes antitachycardia pacing for painless (or relatively painless) termination of monomorphic ventricular tachycardias, programmable low-energy cardioversion, high-energy defibrillation, and backup bradycardia pacing.
A fundamental manner of identifying sustained ventricular tachycardia (VT) involves detecting a heart rate that exceeds a set value, for example, an episode of sustained VT may exhibit a rate in excess of 150 beats per minute (bpm). As already mentioned, other information may be used to distinguish a VT from a supraventricular tachycardia (SVT). Such information can include identification of cycle length stability, abruptness of onset of the tachyarrhythmia, and duration of sustained rate. Regarding ventricular fibrillation (VF), rates in excess of approximately 240 bpm are not uncommon. An ICD, having appropriate defibrillation capabilities can respond to high rates accordingly with defibrillation. Thus, with rates of approximately 150 bpm to approximately 230 bpm, antitachycardia pacing or low-energy may be used; for rates greater than approximately 230 bpm, an ICD device may respond with defibrillation. Of course, the set rates are typically adjustable to account for patient characteristics. Another issue in ICD therapy involves a marked bradycardia following postconversion, which occurs in about 10% of patients successfully converted out of VT or VF. Thus, an ICD device may use backup pacing to prevent and/or minimize postconversion bradycardia.
Regarding antitachycardia pacing, an ICD device may use a technique known as overdrive pacing, which is based on the observation that a mechanism of VT involves re-entry, for example, a circulating wavefront of excitation within a discrete region of myocardium. Often, between the leading edge of the wavefront and the tail of refractoriness, just ahead of the circulating wavefront is a region known as an “excitable gap,” which represents a segment of excitable myocardial tissue about to be depolarized by the propagating wavefront. Hence, the excitable gap circulates with the wave of excitation. According to overdrive pacing, an ICD device delivers a train of rapidly paced pulses to increase the probability of invasion of the excitable gap, for example, antitachycardia pacing can use of a burst of ventricular paced pulses (e.g., a burst of approximately 15 individual pulses). Burst pacing can further involve ramp pacing (e.g., decreasing temporal spacing of individual pulses in a burst with respect to time) and/or scanning (e.g., decreasing temporal spacing between bursts). In general, an ICD device uses antitachycardia pacing as a first tier of therapy, for example, in patients with VT and a heart rate less than approximately 180 bpm. Again, if termination does not occur after first-tier therapy, then the ICD device may implement a more aggressive therapy (e.g., low-energy cardioversion).
In low-energy cardioversion, an ICD device typically delivers a cardioversion stimulus (e.g., 0.1 J, etc.) synchronously with a QRS complex; thus, avoiding the vulnerable period of the T wave and avoiding an increased risk of initiation of VF. In general, if antitachycardia pacing or cardioversion fails to terminate a tachycardia, then, for example, after a programmed time interval or if the tachycardia accelerates, the ICD device initiates defibrillation therapy.
While an ICD device may reserve defibrillation as a latter tier therapy, it may use defibrillation as a first-tier therapy for VF. In general, an ICD device does not synchronize defibrillation therapy with any given portion of an ECG. Again, defibrillation therapy typically involves high-energy shocks (e.g., 0.1 J to 40 J), which can include monophasic or unidirectional and/or biphasic or bidirectional shock waveforms. Defibrillation may also include delivery of pulses over two current pathways.
Overall, antiarrhythmia therapies, such as antitachycardia pacing, cardioversion and defibrillation therapies, can tax an implantable device's resources; however, treatment of the underlying conditions is often imperative because they may be life-threatening. Thus, a need exists for methods and/or devices that can balance these concerns by conserving device resources and/or treating underlying conditions more effectively. In particular, a need exists for methods and/or devices that can help prevent any immediate reoccurrence of arrhythmia after therapy delivery.